Description: The Alps are the archetypical collisional orogenic system on Earth and yet our understanding of processes controlling topographic growth in the Cenozoic remains incomplete. Whereas ideas and models on the Alps are abundant, data from the foreland basin record able to constrain the timing of erosion and sedimentation, mechanisms of basin accommodation and basin deformation are sparse. We combine seismic stratigraphy, micropaleontology, white mica 40 Ar/ 39 Ar, detrital zircon (U-Th)/He and apatite fission track thermochronology to Miocene-Pliocene samples from the retro-wedge foreland basin (Saluzzo Basin in Italy) and to Oligocene-Miocene sedimentary rocks from the pro-wedge foreland basin (Bârreme Basin in France) of the Western Alps. Our new data show that exhumation in the Oligocene-Miocene was non uniform across the Western Alps. Topographic growth was underway since the Oligocene and exhumation was concentrated on the pro-side of the orogenic system. Rapid and episodic early Miocene exhumation of the Western Alps was concentrated instead on the retro-side of the orogen and correlates with a major unconformity in the proximal retro-foreland basin. A phase of orogenic construction is recorded by exhumation of the proximal pro-foreland in both the Central and Western Alps at ca. 16 Ma. This is associated with high sedimentation rates, and by inference erosion rates, and suggests that an increase in accretionary flux associated with the dynamics of subduction of Europe under Adria controlled orogenic expansion in the Miocene.

Description: Marginal-marine deposits are typically excellent hydrocarbon reservoirs around the world. Certain depositional environments may provide natural, selective, physical processes that produce better reservoir rocks than others depending on associated energy, hydraulic regime, etc. With this paper we present new detailed sedimentological, petrographic, and grain-size data of ancient flood-tidal-delta deposits that provide means to characterize this environment in the ancient as well as to determine reservoir characteristics. In this study, we compare grain size, grain and cement composition, and the ratio of pore space to cement from thin sections between tidal, shoreface, and flood-tidal-delta facies of the Sego Sandstone in northwest Colorado. The intent of this comparison is to provide an evaluation of different physical processes and the degree of diagenesis related to different environments to better understand the relative controls on reservoir characteristics. We find that flood-tidal-deltas show a statistically larger average grain size than either shoreface or other tidal facies. Also, flood-tidal-deltas have a smaller ratio of pore-space to cement. Shoreface facies have a larger ratio of pore space to cement overall than that of flood-tidal-delta or other tidal facies. We propose that this may be due to chemistry and higher weathering rates in the backbay environment in contrast to the shoreface environment, where higher mechanical reworking may lead to selective removal of chemically unstable minerals. To test this hypothesis, we conducted statistical analyses of these petrophysical and grain size characteristics. Our results show that flood-tidal-deltas are characterized by clean upper-fine-grained cross-bedded sandstone that is well sorted and subrounded to rounded with a majority of landward paleocurrent directions and a presence of tidal bundles, mud drapes, and reactivation surfaces.

Description: The timing of Tibetan plateau development remains elusive, despite its importance for evaluating models of continental lithosphere deformation and associated changes in surface elevation and climate. We present new thermochronologic data [biotite and K-feldspar 40Ar/39Ar, apatite fission track, and apatite (U-Th)/He] from the central Tibetan plateau (Lhasa and Qiangtang terranes). The data indicate that over large regions, rocks underwent rapid to moderate cooling and exhumation during Cretaceous to Eocene time. This was coeval with 〉50% upper crustal shortening, suggesting substantial crustal thickening and surface elevation gain. Thermal modeling of combined thermochronometers requires exhumation of most samples to depths of

Description: We dated 86 borehole and surface samples from basement-cored Laramide uplifts of the northern Rocky Mountain foreland (Wind River, Beartooth, Bighorn and Laramie Ranges) using the apatite (U-Th)/He system, and eleven samples using the apatite fission-track system (Wind River and Bighorn Ranges). Apatite (U-Th)/He ages generally decrease with increasing subsurface depth (decreasing elevation), and typically range from ∼100 to 50 Ma (Cretaceous to Eocene) within ∼1 km of the surface, to ∼20 Ma (Miocene) and younger ages at depths greater than ∼2 to 2.5 km. Most samples display (U-Th)/He age dispersion ranging from tens to hundreds of Ma, and for some samples we find ages that are older than corresponding fission-track ages. At least one sample per range shows a correlation between apatite (U-Th)/He age and effective U concentration (eU = [U] + 0.235[Th]) of the crystal, indicating that radiation damage has affected He diffusivity, and hence (U-Th)/He age.Forward modeling of simple Laramide-type thermal histories using a radiation damage diffusion model predicts: 1) fossil apatite fission-track partial annealing and apatite (U-Th)/He partial retention zones over similar elevation ranges, 2) (U-Th)/He age dispersion within a fossil partial retention zone up to hundreds of Ma, and 3) (U-Th)/He ages older than fission-track ages within a fossil partial retention zone if eU ≳ 20 ppm. We observe these features in our data from the Bighorn and Laramie Ranges. Most of our samples, however, do not show the correlation between (U-Th)/He age and eU predicted by radiation damage diffusion models. The age dispersion of these samples could be due to the influence of both grain size and eU content, or alternatively due to high U or Th secondary rims around the apatite crystals. (U-Th)/He ages that are older than fission-track ages from Gannett Peak and Fremont Peak in the Wind River Range, and some samples from the Beartooth Range, are most likely the result of He implantation from high eU secondary rims.Best-fit time-temperature paths from inverse modeling of (U-Th)/He age-eU pairs, when extrapolated to other elevations to create model age-elevation plots, reproduce the general distribution and dispersion of (U-Th)/He ages from the Bighorn, Beartooth and Wind River Ranges and suggest that rapid exhumation within the Laramide province likely began earlier in the Bighorn Range (before ∼71 Ma) than the Beartooth Range (before ∼58 Ma). Inverse modeling of borehole data at the northern end of the Laramie Range suggests that the well penetrated a fault sliver at depth. The amount and timing of post-Laramide burial and exhumation cannot be determined from these data.

Description: Interactions between midlatitude westerlies and the Pamir–Tian Shan mountains significantly impact hydroclimate patterns in Central Asia today, and they played an important role in driving Asian aridification during the Cenozoic. We show that distinct westeast hydroclimate differences were established over Central Asia during the late Oligocene (ca. 25 Ma), as recorded by stable oxygen isotopic values of soil carbonates. Our climate simulations show that these differences are present when relief of the Pamir–Tian Shan is higher than 75% of modern elevation (∼3000 m). Integrated with geological evidence, we suggest that a significant portion of the Pamir–Tian Shan orogen had reached elevations of ∼3 km and acted as a moisture barrier for the westerlies since ca. 25 Ma.